Abyssal Drain just needs to be 30s cooldown disjointed from Carve and Spit, healing for something like 600 potency on main target and 200 on the rest as usual. That one change would fix or at least help with many issues DRK has:

1. Large pulls in dungeons would be way more survivable, still not comparable to WAR but WAR is overtuned in dungeons so that's fine.
2. At least some sustain in raids.
3. Increase casts per minute by 2 putting DRK somewhere between GNB and WAR (that DRK, a job focused on oGCD damage, is currently at the same CPM as WAR is tragic).
4. Gives you something to press between 1min/2min cooldowns.
5. A bit of extra damage as DRK is not where it should be in damage department.